grdsample - Resample a grd file onto a new grid
grdsample in_grdfile -Gout_grdfile [ -F ] [ -Idx[m|c][/dy[m|c]] ] [
-Lflag ] [ -Nnx/ny ] [ -Q[value] ] [ -Rwest/east/south/north[r] ] [ -T
] [ -V ]
grdsample reads a grdfile and interpolates it to create a new grdfile
with either: a different registration (-T); or, a new grid-spacing (-I)
or number of nodes (-N), and perhaps also a new sub-region (-R). Inter-
polation is bicubic [Default] or bilinear (-Q) and uses boundary condi-
tions (-L). Note that using (-R) only is equivalent to grdcut or
grdedit -S. grdsample safely creates a fine mesh from a coarse one; the
converse may suffer aliasing unless the data are filtered using grdfft
The name of the input 2-D binary grd file.
-G The name of the output grd file.
No space between the option flag and the associated arguments.
-F Force pixel registration. [Default is grid registration].
-I x_inc [and optionally y_inc] is the grid spacing. Append m to
indicate minutes or c to indicate seconds.
-L Boundary condition flag may be x or y or xy indicating data is
periodic in range of x or y or both set by -R, or flag may be g
indicating geographical conditions (x and y are lon and lat).
[Default uses "natural" conditions (second partial derivative
normal to edge is zero).]
-N Specify number of columns nx and rows ny of new grid.
-Q Quick mode, use bilinear rather than bicubic interpolation.
optionally, append value in the 0 < value <= 1 range. This
parameter controls how close to nodes with NaN values the inter-
polation will go. E.g., a valuefP of 0.5 will interpolate about
1/2-way from a non-NaN to a NaN node, whereas 0.1 will go about
90% of the way, etc. [Default is 1, which means none of the four
nearby nodes may be NaN].
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of wesn. The
two shorthands -Rg -Rd stand for global domain (0/360 or
-180/+180 in longitude respectively, with -90/+90 in latitude).
For calendar time coordinates you may either give relative time
(relative to the selected TIME_EPOCH and in the selected
TIME_UNIT; append t to -JX|x), or absolute time of the form
[date]T[clock] (append T to -JX|x). At least one of date and
clock must be present; the T is always required. The date string
must be of the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or
yyyy[-Www[-d]] (ISO week calendar), while the clock string must
be of the form hh:mm:ss[.xxx]. The use of delimiters and their
type and positions must be as indicated (however, input/output
and plotting formats are flexible).
-T Translate between grid and pixel registration while keeping -R
and -I the same; if input is grid-registered, output will be
pixel-registered and vice-versa. The input file determines -R,
-I and -N so no other options are necessary (except possibly -L
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
If an interpolation point is not on a node of the input grid, then a
NaN at any node in the neighborhood surrounding the point will yield an
interpolated NaN. Bicubic interpolation [default] yields continuous
first derivatives but requires a neighborhood of 4 nodes by 4 nodes.
Bilinear interpolation [-Q] uses only a 2 by 2 neighborhood, but yields
only zeroth-order continuity. Use bicubic when smoothness is important.
Use bilinear to minimize the propagation of NaNs.
To resample the 5 x 5 minute grid in hawaii_5by5_topo.grd onto a 1
grdsample hawaii_5by5_topo.grd -I1m -Ghawaii_1by1_topo.grd
To translate the gridline-registered file surface.grd to pixel regis-
grdsample surface.grd -T -Gpixel.grd
gmt(l), grdedit(l), grdfft(l), grdfilter(l)
GMT4.0 1 Oct 2004 GRDSAMPLE(l)
Man(1) output converted with